The present invention relates in general to methods for manufacturing a composite material structure, in which on a precured element (skin) are glued other uncured elements (beams) by placing an adhesive layer between them (between the precured element and each uncured element), thereby obtaining a structural union. The adhesive is cured at the same time as the latter elements.
More specifically, the object of the invention is to develop the required theoretical concepts and the corresponding manufacturing methods for providing a union system by co-bonding of one or several elements (beams) made from composite materials and uncured, and a base (skin) also made of composite material but which is precured, with multiple changes in thickness. A precise adjustment must be obtained of the uncured elements, both with the adhesive surface (skin) and with the other, upper surface.
For this purpose, the tooling used is the most relevant factor, which is in this case a rigid invar rod (described in detail below) with a direct bag that allows to obtain a high dimensional precision at the same time as a tight positioning tolerance. To clarify the term xe2x80x9cdirect bagxe2x80x9d, it should be pointed out that the direct vacuum bag concept relates to the fact that the elements comprising the vacuum bag (FEP or fluoro-ethylene-propylene, AIRWEAVE type aerator and bag plastic) are directly on the part to be cured without any interposed tooling. This ensures a uniform consolidating pressure.
The union is achieved by curing the adhesive layer under strict pressure conditions and at its polymerization temperature, which must match that of the resin of the uncured elements as both chemical processes take place simultaneously in the same autoclave cycle.
Likewise, the union effected is designed to withstand shear loads applied to the skin by the beams, due to deflections of the structure, and detachment forces applied on the beams by the skins, as well as various types of internal pressures such as those of a fluid when the torsion box is the fuel tank.
The most remarkable characteristic of the present invention is the use of a rigid tooling (a system of rigid tools and rakes) for the bonded union, combined with applying autoclave pressure using a scheme with a vacuum bag in direct contact with the elements to be bonded and cured.
In order to bond the uncured elements to a precured skin which must match another complex surface at the unbonded end, a manufacturing system was initially developed with a flexible tooling using the xe2x80x9cinflatable toolxe2x80x9d technique. These tools were made from an elastomer material stiffened as required with carbon fiber.
The high cost and low reliability of this tooling spurred the development of a rigid tooling system to solve these problems; this is the co-bonding system with rigid tools.
During the development stage of the rigid tooling trials were performed with tools of various configurations:
Several configurations were tested with steel material, which were discarded because of the thermal gradients generated which resulted in deformations of the part to the point of not obtaining the required quality.
Two constructive solutions have been tested using invar:
Rigid tools made from welded sheets which are later machined. This solution is the lightest but its construction is extremely complex and involves several deformation and straightening operations during fabrication.
It is also possible to leave a small wall thickness after machining, with the resulting risk of collapse of the tool in the autoclave. The resulting weight does not allow manual handling.
Rigid tools made from a sheet with a sufficient thickness and enlightened by machining, and later covered by a welded plate.
The enlightened material weighs ≈25 kg as compared to a weight of the solid tool of around 150 kg. This enlightening is not justified due to handling issues as it greatly increases the tool fabrication cycle and its handling still requires additional means.
As well as the use of different materials and configurations of the rigid tools, another basic aspect in the use of this type of blade-shaped rigid tooling is the distance between the edge of the rigid tool and the radius of the beam foot. The following configurations were tested:
The rigid tool extending 2 mm into the radius.
The rigid tool remaining 2 mm above the radius.
The rigid tool extending as far as the middle of the radius.
It was concluded that the rigid tool should end above the radius of the foot, as this configuration provides the best dimensional and quality results, as well as facilitates demolding.
Later studies led to an optimization of the distance between the rigid tool and the beam foot radius, arriving at the conclusion that the ideal distance was 3 mm from the edge of the rigid tool to the start of the beam foot radius.
The results obtained indicate that rigid tools should be made of solid invar, as this simplifies their construction and improves dimensional tolerance. Additionally, they are handled in all cases with auxiliary means and not manually, regardless of their configuration.
As regards the bonded unions, using a different type of tooling, the prior art closest to the application are those relating to:
1. Joining beam stiffeners of the torsion box for the A330-340 airplane horizontal stabilizer (currently in the production stage).
2. Joining the longitudinal stiffeners for the skin of the torsion box of the CASA 3000 airplane wing (in prototype stage).
3. Joining auxiliary longitudinal beams to the skin of the torsion box of test FB.5-1 of the technological development program for large airfoils (GSS) to be applied to the horizontal stabilizer of the A3XX.
From the results of the above experiences and from other relevant manufacturing studies and tests it was concluded that the application of the method of the present invention is feasible and reliable for its use in parts of highly demanded withstanding structures and with high quality requirements, with complex shapes and strict dimensional tolerances.
This invention is applicable to the manufacture of structures made of composite materials in which participate a precured element (skin) and other uncured elements (beams) that are cured simultaneously to their union to the precured element.
The structures for which this technique would be applicable are such as:
Airplane structures and controls, such as airfoils, moving airfoil surfaces, fuselages.
Space ships
Marine and land vehicles
Industrial machinery and equipment.
The various manufacturing stages which comprise the full process are:
Fabrication of the Skin
Tape laying on a curved tool.
Placing the vacuum bag on a laminate.
Curing in an autoclave.
There is no demolding operation nor a non-destruction inspection.
Fabrication of the J-beams
Flat tape laying.
2D cutting in fresh state on patterns.
Mounting patterns until final configuration of the beam cloths.
A first hot forming cycle to obtain two L-shaped beam halves.
Placing one half on the other.
A second hot-forming cycle, to provide the final J-shaped beam.
3D cutting of the uncured beam rises as well as other cutting to obtain the final size of the beam after the curing cycle.
Fabrication of the Vacuum Bag
Approximate flat layout of the final bag configuration.
Tracing the bag in a flat machine with numerical control or manually with jigs or Mylar. The position of the beams and fasteners on the radii is traced.
Formation and manual attachment of the fasteners.
Fabrication of the Final Structure: Co-bonding
Assembling the beams on rigid invar tools on auxiliary preassembly benches. Each bench has two rigid tools to allow ergonomic working conditions.
Placing all possible elements of the final vacuum bag on the beams in the preassembly benches. Additionally, a consolidation is carried out to ensure adjustment on the skin. For this, the preassembly benches are provided with a surface which perfectly resembles the surface of the skin.
Transfer of rigid tools+rakes+beams to their final position on the skin.
Placing the remaining elements of the vacuum bag.
Assembling the prefabricated and checked vacuum bag.
Final adjustment of the vacuum bag with the assembly in a vertical position for large surfaces with difficult access to certain areas.
Autoclave curing cycle.
Demolding.
Non-destructive inspection of the skin.
Re-edging (only for the skin as the system of rigid tools allows to obtain beams with their final geometry).
Non-destructive inspection of beams.
Priming and painting.
Materials
The materials to be used will be composite materials, in which the fibers and resin can be:
Fibers
Carbon fiber.
Glass fiber.
Ceramic fiber.
Aramid fiber.
Boron fiber.
Resins
Epoxy resin.
Thermoplastic resin.
Other thermosetting resins.
The object of the invention is a method for manufacturing composite material structures in which several uncured elements (beams) are joined to a precured element (skin) so that the union has structural requirements.
The bonding and curing of the beams is achieved by a prior forming and a final curing in an autoclave with a direct vacuum bag.
The uncured elements have a J-shaped cross section.
The basis of the manufacturing method is the optimized design of forming tools (made of aluminum and improved wood with an integrated vacuum system for overturning) and particularly curing in an autoclave, rigid tools made of invar (to avoid deformations due to thermal expansion) and the automation of all processes.
The method is applicable to any base structure which must be stiffened by elements with a precise geometry.
The tape laying technique can be either manual or automated, although the automated tape laying system optimizes the process considerably.
In a specific embodiment the invention discloses a method for manufacturing precured parts of composite material by using uncured J-beams, in which are structurally joined at least two parts made of composite materials, of which a first part known as the base part or skin is in a cured state and a second part or parts, known as beams, are uncured, and in which the two parts are joined by a layer of structural adhesive so that the second part is compacted against the first, with a suitable cross linking of the resin of the composite material, and so strongly bonded to the skin of the first part that the required strength of the adhesive layer is ensured. This method is characterized by the following stages: laminating superposed layers of preimpregnated composite material so that the fiber orientation is adapted to the structural requirements of the part to be obtained, obtaining from the resulting laminates on one hand the base part and on another a set of basic stacks used to form the second part; curing the base part in an autoclave; cutting the flat laminate with the areas of different thickness from which the second parts are obtained; assembling packages from the patterns obtained in the previous cutting; hot forming in two cycles, by applying heat and vacuum, of the previously obtained flat configurations to obtain a preform with a J-shaped cross-section; mounting the preforms on the curing tools on auxiliary preassembly benches which simplify this task; precise positioning of all tooling (rigid tools+rakes) and J-shaped parts on the precured base; mounting a previously made and checked vacuum bag; overturning the part and the tool to a vertical position when the parts have a large area and are difficult to access, in which position the fine adjustment of the vacuum bag is performed; and performing the autoclave curing cycle.
In accordance with the invention a base part and one or more second parts are joined to obtain a finished precured part. The uncured elements to be bonded are obtained from flat laminates of varying thicknesses in some areas, which are later cut and stacked in packages until the final configuration of the part, with packages of at least two cloths being stacked and in no case with two cloths touching each other.
Likewise, the uncured elements to be bonded are hot formed to obtain preforms with the final geometry, so that they can be easily mounted on the curing tools (rigid tools). The hot forming tools are made of aluminum with improved wood on their top part, which is in contact with the fiber, in order to prevent heat transfer losses as well as in the integrated vacuum system for overturning said tools.
In addition, the curing tools generally have a rectangular trapezoid cross-section so that the geometrical quality of the part is ensured, allowing to adjust the beams on their top surface with another part of the type of the base part. These curing tools are made of invar to prevent deformations due to thermal expansion during the autoclave cycle.
Furthermore, between the edge of the rigid tool and the foot radius of the beam there is a 3 mm separation which ensures the geometrical quality of the part as well as facilitates the demolding; the autoclave curing process is performed at a pressure between 585 kPa and 896 kPa, and at a temperature of up to 190xc2x0 C. depending on the composite material used, with a heating gradient of 0.5 to 2xc2x0 C./min.
With the method of the invention parts are obtained that can be applied in structures and controls of aerospace, marine and land vehicles, as well as in industrial machinery and equipment. Specifically, the base part (skin) comprises the skin of an airplane wing, a stabilizer or any other element which must be stiffened to fulfill its structural functions.
In accordance with the invention, the uncured parts have a J-shaped cross section and thickness between 1 and 6 mm, while the base part has a length of up to 7 m and is shaped as a delta.
The vacuum bag used in the method of the invention is quite large, so that it is traced with a numerical control machine and made before it is placed.
The composite material used in the method of the invention consists of fibers and resins selected among glass fiber, carbon fiber, aramid fiber, boron fiber, epoxy resin, thermoplastic resin and other thermosetting resins.